The present invention relates to a process for forming a multilayer connection in the fabrication process of a semiconductor device, and particularly, to a process for fabricating a multilayer connection for a device having a contact hole or a via hole connecting an upper layer connection and a lower layer connection, where a metal plug is formed by means of blanket metal technology.
With the increasing integrating density of semiconductor devices, the demand for higher density connection patterns requires the patterning technology to implement a still finer and multilayered connection. Accordingly, the technique for forming multilayer connection is increasing its importance in the process for fabricating semiconductor integrated circuits.
In case of forming the multilayer connection above an aluminum connection pattern is formed as a first layer on the portion of an active element such as a transistor formed on a silicon substrate, with an electrode formed therebetween. Another aluminum connection pattern is formed as a second layer on the resulting first layer with an interlayer insulation film such as a dielectric film interposed therebetween. The connection patterns thus provided as the first and the second layers are electrically connected with each other by a metal plug, which is obtained by filling, with a metallic material, an aperture portion formed in the interlayer dielectric film, such as a contact hole or a via hole. In case of a multilayer connection structure having three layers or more, a stacked contact structure is employed so that the contact holes are aligned with each other to establish superposed plugs for simplifying connection patterns and reducing connection resistance.
When an interlayer dielectric film is formed on the lower layer connection, however, steps are formed on the interlayer dielectric film in accordance with the lower layer connection pattern. With increasing fineness and with progressing multilayered structure of the connection, the steps increase their height and become steeper as to impair the process precision of the upper layer connection pattern and the reliability of patterning. In case of an aluminum connection, in particular, not much can be expected at present in improving the coverage of the steps. It is therefore required to increase the planarity of the interlayer dielectric film. The necessity of improving the planarity is important from the viewpoint of the depth of focus degrading with decreasing wavelength in lithography. It can be seen therefore that, from the viewpoint of increasing precision, quality, and reliability of a semiconductor device, the problem of step formation is an important one to be solved.
Another problem is the difficulty of forming a highly reliable metal plug on a fine contact hole or via hole having a high aspect ratio (i.e., elongated holes). Various technologies for forming insulating films, flattening, and metal plugging have been developed heretofore to overcome the problems above. In the process of multilayer connection, in particular, metal plug technology has attracted much attention as a key technology for filling a connection material into fine contact holes and the like. Metal plug technology includes developed techniques such as blanket tungsten (W) technique, selective metal growth technique, high temperature aluminum sputtering, and aluminum reflow process. Among the techniques enumerated above, in particular, blanket metal techniques such as blanket W technique and the like are put into practice for mass production because of their stability despite the process is somewhat complicated. A blanket metal technique comprises forming a metallic film such as a tungsten film and the like on the entire surface of an interlayer dielectric film inclusive of the contact hole, and then removing it by etch-back process and the like while leaving the metal in the contact hole. In this manner, metal plugs can be formed inside the contact holes.
In the blanket metal technique above, steps can be generated due to the depressions that are formed in the contact hole portions. This problem is known as plug loss. Referring to FIGS. 6A to 6C, the problem of plug loss is described in further detail below. In FIG. 6A, a lower layer connection 101 is formed on a substrate (not shown in the figure), and an interlayer dielectric film 102 as an interlayer insulation film is formed thereon. Then, after forming an aperture portion 103 which provides a contact hole in the interlayer dielectric film 102, a glue layer 104 comprising an ohmic layer (e.g., a titanium layer) and a barrier layer (e.g., a titanium nitride (TiN) layer) is formed inside the aperture portion 103 and the interlayer dielectric film 102. A metallic material such as tungsten is applied to the entire surface of the glue layer 104 to provide a blanket metal layer 105. Referring to FIG. 6B, the blanket metal 105 and the glue layer 104 provided on the interlayer dielectric film 102 are removed by etch-back using reactive ion etching process (RIE) and the like to form a metal plug 106 made of the metallic material for the blanket metal layer 105 left over inside the aperture portion 103. To completely remove the layers 104 and 105 from the entire upper surface of the interlayer dielectric film 102 without being affected by the non-uniformity in film thickness and in etching, the etch-back process is performed in a so-called over etching manner by continuing etching for some time even after the interlayer dielectric film 102 is exposed. Thus, the upper surface of the metal plug 106 in the aperture portion 103 is faster etched to become a depression 109 that has a level difference 107 with the upper surface of the interlayer dielectric film 102.
When an upper layer connection 108 is formed on the interlayer dielectric film 102 having the depression 109 as described in the foregoing, as is shown rather exaggerated in FIG. 6C, an indentation 110 is formed in the upper layer connection 108 on the depression 109. Thus, inferior coverage results on the upper layer connection due to the indentation 110, which leads to unfavorable effects such as an increase of the wiring resistance or a decrease in connection reliability. In the aforementioned stacked contact structure, the problem of plug loss is found to be particularly serious as to affect the reliability of the entire device, because the contact hole portions are superposed one after another.
Accordingly, an object of the present invention is to overcome the aforementioned problems of the metal plug technology, and to provide a process for fabricating a multilayer connection which enables a highly reliable device improved in the coverage of the upper layer connection. More specifically, the present invention provides a process for fabricating a multilayer connection by overcoming the problem of forming a depression due to overetching in forming a metal plug, and thereby flattening the surface of the interlayer dielectric film.